Stilly19

My car is a 2003 ford mustang gt. I upgraded my stereo system about 8 months ago. I installed a AVIC-U310BT by pioneer. and i also had Pioneer speakers installed. I installed an amp in my trunk also with sub capabilities. the mid range speaker on the passenger side door blew recently. and instead of just replacing it i want to upgrade to a better system all around. I want to get better mid range speakers installed in the door panels. Any suggestions? I also want to install a sub in my trunk that would go againts the back of the trunk or two smaller subs that would be on the left and right sides. Are there any retailers that have pre-made boxes that would fit this car? Someone also suggested putting a small sub box underneath the passenger seat, would this be a good idea? also what would be some prices for all this. I dont want to spend money on high end quality. What would it be for mid-high range quality?